Fundamental aspects of creep in metal matrix composites
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I.
INTRODUCTION
The creep behavior of metal matrix composites (MMCs), in which ductile metallic matrices are reinforced with rigid ceramic phases, has become a topic of considerable interest in recent years, primarily because these materials have a potential for use in structural applications at elevated temperatures. From these extensive investigations,[1–16] an overview has emerged in which the creep behavior of MMCs is often characterized by high and variable values for the apparent stress exponent na (5 ] ln εz /] ln s, where εz is the creep rate and s is the applied stress) and the apparent activation energy Qa (5 2R.] ln εz /] ln (1/T ), where R is the gas constant and T is the absolute temperature). By making an assumption that creep occurs under an effective stress defined as (s 2 s0), where s0 is a threshold stress representing a lower limiting stress below which creep cannot occur,[17] it is generally found that the creep behavior of MMCs may be rationalized in terms of values of the true stress exponent (n) and the true activation energy (Q) which are close to those reported for the creep of unreinforced pure metals and solid solution alloys. Although significant progress has been made in delineating the creep behavior of MMCs at high temperatures, several fundamental issues require further consideration. For example, it is important to consider whether true steady-state conditions occur in the creep of MMCs, and detailed analyses and microstructural observations are needed to more fully evaluate the precise deformation mechanisms responsible for the measured values of n, Q, and s0 in different MMCs. The objective of this article is to address these fundamental aspects and to provide a better understanding of the flow characteristics and the deformation mechanisms occurring in the creep of MMCs.
YONG LI, Research Assistant Professor, and TERENCE G. LANGDON, Professor, are with the Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453. This article is based on a presentation made in the symposium ‘‘Fatigue and Creep of Composite Materials’’ presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
ANALYSIS AND DISCUSSION
A. The Creep Curves in MMCs When MMCs are tested under creep conditions using tensile or double-shear configurations, it is usually possible to identify the standard three distinct stages of flow within the creep curves;[2,5,7,15,16,18,19] thus, there is generally a normal primary stage in which the strain rate decreases with increasing time, and this is followed by a brief secondary stage where the strain rate remains reasonably constant, and then an extended tertiary stage follows where the strain rate increases with time. In many experiments on MMCs, the secondary stage is of a sufficiently short duration that it is best described as a minimum creep rate, which is observed most
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